Many applications are known for carriers and modifiers of radiation. Optical radiation, for example, can be contained by optical waveguides (e.g., optical fibers) and guided from one source to another. Lenses are often used for focusing optical radiation. One desirable characteristic of many of such devices is their ability to contain the radiation, i.e., to minimize the amount of the radiation that escapes from the desired location to undesired locations. A medium that better contains the radiation is more efficient.
Many polymers are physically changed when radiation is applied thereto. For example, the optical index of refraction of a photopolymer changes when it is exposed to optical radiation, i.e., light. However, usual changes in index of refraction based on this kind of "photopolymerization" exhibits a relatively slow response to optical intensity changes. For example, light-induced changes in index of refraction in a polymer may take on the order of milliseconds to complete.
Throughput, however, is often very important in production of these devices. The prior art has sometimes illuminated the photopolymer with intense light for a short time, to increase the fabrication throughput. The illumination is often turned off before the index of refraction change or any nonlinear optical effects appear. This allows the devices to be made at the maximum speed possible.
One aspect of the present invention goes against this established teaching by using relatively long exposures and inducing an index change in the material of interest during the exposure. Unexpectedly, the materials and operations described herein enable the index change to change the radiation passing properties of the material in a way that tends to contain the radiation that causes the index change. Two different mechanisms are described herein: self-trapping and self-focusing.
The field of high resolution projection photolithography has been limited by the resolution and depth-of-focus during the stage where patterns are formed. This in turn limits the integrated circuit density that can be produced using these patterns.
The smallest feature size x.sub.min that can be projected by a coherent imaging system is ##EQU1##
and the depth-of-focus (DOF) is ##EQU2##
where .lambda. is the wavelength of the illumination and N.A. is the numerical aperture of the optical projection system.
A direct route to patterning smaller features photolithographically is to reduce the wavelength from the i-line standard of today (365 nm) to excimer laser wavelengths (248 or 193 nm). The N.A. is typically 0.5, so the minimum feature size is on the order of the exposure wavelength.
However, many have desired to use even smaller features, e.g., a 0.1 .mu.m (=100 nm) process. At this point, the resolution and depth-of-focus constraints of conventional optical lithography become severe. Therefore, techniques to extend the performance of optical lithography into this regime are of particular technological and economic significance.
It is hence desirable to reduce the wavelength of the optical beam, for example, to less than 248 or 193 nm. Light sources capable of emitting such short wavelengths and compatible photoresists are uncommon and costly. Therefore, conventional wisdom has implied that the resolution and depth-of-focus determined by the shortest practical optical wavelength (193 nm) has placed a fundamental limit on further miniaturization of integrated circuits using conventional optical lithography. There is a need for techniques that will enable making smaller features.
In view of this need, the present invention describes new techniques to extend the performance of photolithography by properly tailoring the nonlinear optical response of the photoresist in tandem with the optical exposure. The present invention shows exposure techniques exploiting self-focusing and self-trapping. These techniques provide significant improvement in the resolution and depth-of-focus above the inherent values of the optical projection system, even without replacing the existing exposure tools. These techniques could be used in conjunction with improved exposure tools, e.g., lasers having wavelengths less than 200 nm, or x-ray exposure tools, for even better response.
Self-focusing and self-trapping are two examples of nonlinear optical effects which may arise from one of many physical mechanisms. Self-focusing describes the formation of a light induced channel in an illuminated material which confines the optical beam. This channel serves as a lens. Self-trapping occurs when self-focusing substantially exactly counteracts beam spreading due to diffraction. When this happens, the cross section of the light induced channel remains substantially constant with propagation distance over the distance of the self-trapping. Other similar mechanisms also exist. For example, a modified self-trapping effect occurs when self-focusing is somewhat less than beam spreading due to diffraction. In that case, the change slows the rate with which diffraction occurs.
A material exhibits self-trapping or self-focusing when the index of refraction changes in the presence of optical radiation in a way to induce waveguiding of that same optical beam which causes the index of refraction to change.
The conditions under which a beam is self-trapped may be quantified. If, for instance, the Gaussian beam waist is .omega..sub.0 the beam expands by diffraction in a homogeneous medium having an index of refraction n.sub.0 with an angular divergence given by: ##EQU3##
where .lambda. is the wavelength of light in vacuum, and .theta..sub.1 is an angle describing the divergence of the beam relative to normal. Beam spreading by diffraction is avoided if the angular envelope given by Eq. 3 is trapped within the waveguide by total internal reflection. That is, light of a certain optical mode will be guided through a medium along a waveguide distinguished by a region of larger index of refraction relative to the surroundings.
In an optical material of this type, the waveguide is called the core, and the surroundings is called the cladding. The guidance condition for an optical waveguide of radius r and index n.sub.0 depends on the index difference An between the core and cladding, as quantified by the following relation: EQU k r(2n.sub.0.DELTA.n).sup.1/2.gtoreq.2 (4)
Self-trapping occurs when this waveguide is formed by the same beam which is trapped by the waveguide. Upon trapping, the beam diameter remains nominally constant, independent of the propagation distance.
In general, the diameter of a trapped beam may be slightly modulated along the propagation direction, as if waveguiding by the medium were due to a periodic sequence of convex lenses. This results in a channel with diameter variations. In this case, self-focusing does not exactly balance diffraction point-by-point along the longitudinal direction. Nevertheless, on average, the beam is trapped.
In the prior art, self-trapping and self-focusing have only been observed for a highly restrictive set of material and exposure conditions which many of ordinary skill in the art have believed to be impractical. Waveguiding based on the Kerr effect vanishes when the beam intensity is reduced below a critical value on the order of megawatts per square cm, as reported in Chiao et al., Phys. Rev. Lett., 13, 479-482 (1964). This critical value is typically so large (MW cm.sup.-2) that self-focusing and self-trapping are difficult to observe with common lasers. Self-trapped beams have also been observed by using the Kerr effect in sodium vapor and in glass.
Alternately, self-trapping has been achieved by two-wave mixing in photorefractive crystals using low intensity optical beams, as reported in Segev, et al., Phys. Rev. Lett., 68, 923-926 (1992) and Duree, et al., Phys. Rev. Lett. 71, 533-536 (1993). Self-trapping by the photorefractive effect requires only low optical intensities (about 10 mW cm.sup.-2). This guiding behavior arises from a fundamentally different physical mechanism than the Kerr effect. Namely, diffraction is balanced by two-wave mixing among the spatial frequency components of the input beams.
Self-focusing and self-trapping demonstrated in the prior art hence either has required very high optical intensities (for the Kerr effect), or very specialized materials, e.g., inorganic crystals. In these materials, unlike the polymers that are the subject of the present invention, light does not induce an attendant structural change that can be used to define three dimensional structures, such as those in photoresists.
Photopolymers find application among a wide gamut of technologies. For instance, the fabrication of microelectronic integrated circuits often relies on photopolymerization to form resist structures of precise and small dimensions, which are subsequently transformed into circuit elements following additional processing steps. It is an object of the present invention to describe techniques and structure which allow exploiting the self-trapping and self-focusing phenomena in photopolymers.
The present inventors have discovered that nonlinear optical effects occur at low optical intensities in combination with structural changes--For instance, the transformation of a liquid to a solid. This is also used according to another aspect of the present invention to enhance the practical importance and applications of self-focusing and self-trapping.
The propagation of light through an optical material is determined by knowledge of the real n' and imaginary n" parts of the complex index EQU n=n'+i n". (5)
A common nonlinear optical phenomenon in the prior art takes advantage of a light induced change in the imaginary part of the index of refraction (equivalently absorption) upon exposure. For instance, contrast enhancement layers (CEL) formed of a photobleachable dye are often overcoated on a photoresist to improve the contrast of the projected image as it passes through the layer [S.V. Babu et al., J. Vac. Sci. Technol. B 6, 564-568 (1988), S. V. Babu, E. Barouch, J. of Imaging Science 33, 193-199 (1989), B Davari et al., IEEE Transactions on Electron Devices 39, 967-975 (1992), M. Endo et al., J. Vac. Sci. Technol. B 6, 87-90 (1988), B. F. Oriffing, P. R. West, in Solid State Technology 1985), pp. 152-157, Y. Hiral et al., J. Vac. Sci. Technol. B 5, 434-438 (1987), D. C. Hofer et al., "Characterization of the induction effect at mid-ultraviolet exposure: application to AZ2400 at 313 mn," Optical Microlithography-Technology for the Mid-1980's (SPIE, 1982), vol. 334, pp. 196-205, K. Kaifu et al,, J. Vac. Sci. Technol. B 5, 439-442 (1987), W. Loong et al., J. Vac. Sci. Technol. B 8, 1731-1734 (1990), T. Ochiai et al., S. Photochem. Photobiol A: Chem. 65, 277-284 (1992), Y. Tomo et al., Polymer Engineering and Science 29, 902-906 (1989). The CEL introduces exposure thresholding through absorption photobleaching. This improves the contrast of the projected image, and has been incorporated into a 0.25 .mu.m i-line process [B. Davaii et al., IEEE Transactions on Electron Devices 39, 967-975 (1992)].
The present inventors have realized that using an optical nonlinearity arising from the real part of the complex index of refraction produces a new result. The real part of the complex index of refraction will be simply called the index of refraction, as is conventional in the art.
When an appropriate liquid photopolymer is illuminated with UV radiation, the exposed regions crosslink to form a solid with a real index of refraction which is typically 0.1 to 0.04 larger than the liquid. This is equivalent to a polymerization reaction. A subsequent development stage removes the unexposed polymer. This operation is often conventionally called a "negative" exposure process in the art.
Alternately, when a solid crosslinked photopolymer is exposed to appropriate radiation, crosslinks may be broken. This is similar to a depolymerization reaction, and is typically called a "positive" exposure process in the art. A subsequent development stage removes the exposed polymer.
The present inventors have found that both of the polymerization and depolymerization chemical reactions lead to changes in the index of refraction upon optical exposure which can produce self-focusing and self-trapping under the right material and exposure conditions as described herein.
The descriptions given herein focus primarily on the index change associated with polymerization reactions. However, it should be understood that depolymerization may be used interchangeably.
As described above, the present invention provides a nonlinear optical method of increasing the resolution and depth-of-focus in projection photolithography above the fundamental limits inherent in conventional linear optics. The invention further provides techniques for forming polymeric microstructure from photosensitive polymer precursor materials by exploiting self-focusing and self-trapping during the exposure stage.
The exposure of photosensitive compositions to radiation alters their chemical structure. The alteration is usually effected through bond cleavage and bond formation. The index of refraction of polymers can increase or decrease upon illumination to radiation of appropriate wavelength. The preferred materials described herein exhibit a property whereby the index of refraction of the respective precursors changes upon exposure to radiation. The characteristics of the exposure are chosen such that the change in index upon polymerization/depolymerization necessarily occurs during the exposure rather than subsequent to the exposure.
The radiation for photopolymerization is preferably optical, e.g., infra red, visible light or ultra violet. It may be incoherent (as produced by a light bulb), partially coherent (as produced by an excimer laser) or coherent (as produced by an ideal laser) and may be intensity modulated in time. Radiation may be directly incident onto a photosensitive composition or may be directed through a radiation transparent container into a photosensitive composition.
One aspect of the present invention provides a technique to reduce or eliminate diffractive spreading of a high resolution image by inducing a barrier to diffraction in the form of a light channel within the photopolymer. In the case of self-focusing, the cross section of this light channel decreases with propagation distance, while in the case of self-trapping, the cross section of this light channel remains substantially constant. More generally, however, the radiation induced light channel counteracts at least some of the diffraction effects which would otherwise exist.
In another aspect of the present invention, a layer of a photosensitive composition is exposed to radiation, thereby forming a region of changed index of refraction in the layer upon which radiation is absorbed. In the case of a negative exposure process, the polymerization reaction leads to the formation of a polymer with increased molecular mass. In the case of a positive exposure process, a depolymerization reaction leads to the formation of a polymer with reduced molecular mass. The spatial profile of the increase in index of refraction has a form defined by the intensity pattern of the radiation incident on the layer.
Preferably, the index of refraction for the preferred materials used herein should increase by at least 10.sup.-3 upon exposure, although the present invention may be usable with materials that are two orders of magnitude less sensitive than this. This change in the index of refraction modifies the propagation of radiation deeper in the material. That is, the radiation propagates effectively through a channel, inducing further polymerization in an adjacent region deeper within the photosensitive composition. As the radiation propagates, the channeling effect counteracts diffraction to varying degrees. For a particular set of material and exposure conditions, self-focusing closely balances diffraction, and the optical beam travels through the light induced channel with a substantially constant cross section.
In an alternate embodiment, the diameter of a light channel at a given point along the channel can be altered by modulating the optical intensity, wavelength, or intensity profile at that point during photopolymerization. These light channels also serve as optical fibers of modulated diameter which can be used for optical data storage or as optical filters, for instance.